This invention relates, in general, to fluid samplers, and more particularly to apparatus for performing non-destructive collection of fluid samples from subsurface earth formations traversed by a borehole.
The sampling of fluids contained in subsurface earth formations provides a method of testing formation zones of possible interest by recovering a sample of any formation fluids present for later analysis at the earth's surface while causing a minimum of damage to the tested formations. Thus, the formation sampler is essentially a point test of the possible producibility of subsurface earth formations. Additionally, a continuous record of the sequence of events during the test is made at the surface. From this record valuable formation pressure and permeability data can be obtained for formation reservoir analysis.
Early formation fluid sampling instruments, such as the one described in U.S. Pat. No. 2,674,313, were not fully successful as a commercial service because they were limited to a single test on each trip into the borehole. Later instruments were suitable for multiple testing; however, the success of these testers depended to some extent on the characteristics of the particular formations to be tested. For example, where earth formations were unconsolidated a different sampling apparatus was required than in the case of consolidated formations.
One major problem which has hampered the reliable testing of subsurface earth formations has been in designing a suitable system for preventing seal loss between an extensible packer element of the formation tester instrument and the formation at the initiation and during fluid sample collection. This problem is particularly acute in highly unconsolidated formations. A related problem has been in designing a system for eliminating the sudden pressure drop within the formation fluid sample line when the control valve controlling the fluid sample collection tank is opened. This sudden pressure drop can result in degeneration of the formation in the packer area causing a loss of seal between the packer and the formation resulting in contamination of the formation fluid sample.
In an effort to control the rate of fluid sample intake, and thus reduce the chances of packer seal loss, U.S. Pat. No. 3,022,826, issued to Kisling III, attempts to overcome the problem by employing a flexible bag member as a fluid sample collection chamber and by pressure balancing the flexible bag to reduce the rate of fluid sample intake. Another technique for controlling the rate of fluid sample intake can be found in U.S. Pat. No. 3,653,436, issued to Anderson et al, which continuously employs formation pressure to slidably move a flow restricting cover from a position within the sample intake probe. As the flow restricting cover moves rearward within the probe a filter screen is gradually exposed allowing formation fluid flow into a sample collection tank. Yet another system for controlling initial flow rate is disclosed in U.S. Pat. No. 3,780,575, issued to Urbanosky, where the flow restriction is controlled by a pressure ratio of borehole pressure to formation pressure, rather than simply based on formation pressure, as in Anderson et al. While these designs represent improvements, usage has shown them to be less than totally successful particularly in highly unconsolidated formations.
Accordingly, the present invention overcomes the deficiencies of the prior art by providng method and apparatus for obtaining a plurality of formation fluid samples under adverse formation conditions in a single traversal of the borehole.